As illustrated in FIG. 1, a hydraulic circuit of an excavator, to which the present invention is applied, includes a hydraulic pump 1 connected to an engine (not illustrated); an attachment 2, such as an arm, connected to the hydraulic pump 1 and a traveling motor 3; an operation lever (RCV lever) 4 and a traveling pedal (RCV pedal) 5 outputting a control signal in proportion to an operator's operation amount; a main control valve (MCV) 6 installed in a flow path between the hydraulic pump 1 and the attachment and a flow path between the hydraulic pump 1 and the traveling motor 3 to control a start, stop, and direction change of the attachment 2 and the traveling motor 3 when a corresponding spool is shifted; and a controller (ECU) 7 controlling a discharge flow rate of the hydraulic pump 1 according to an operation of the operation lever 4 or the traveling pedal 5.
In the excavator having the above-described hydraulic circuit, pilot signal pressure, which is supplied from a pilot pump (not illustrated) when an operator operates the operation lever 4, shifts a spool 8 for an attachment of the main control valve 6 through the operation lever 4. Through this, hydraulic fluid that is supplied from the hydraulic pump 1 is supplied to a boom cylinder through the spool to drive an attachment, such as a boom, an arm, or a bucket.
Further, if the operator presses the traveling pedal 5, the pilot signal pressure shifts the traveling spool 8 of the main control valve 6 through the pedal 5. Through this, left and right traveling motors are driven by the hydraulic fluid that is supplied from the hydraulic pump 1.
On the other hand, if the operation lever 4 or the traveling pedal 5 is operated and shifted to a neutral position in order to temporarily stop the traveling or operation of the equipment, the supply of the pilot signal pressure to the corresponding spool of the main control valve 6 is intercepted, and thus the spool returns to the neutral position. Through this, the hydraulic fluid that is discharged from the hydraulic pump 1 returns to a hydraulic tank 10, and thus the attachment such as the arm becomes unable to be operated.
On the other hand, in the case where a stick of the spool of the main control valve 6 occurs due to foreign substances or other damages, the spool of the main control valve 6 is kept in a shifted state even if the operator shifts the operation lever 4 to the neutral position to stop the operation of the equipment. Due to this, the flow path between the hydraulic pump 1 and the attachment 2 is kept open, and thus the hydraulic fluid that is discharged from the hydraulic pump 1 is continuously supplied to the attachment 2 along the flow path. Accordingly, the attachment of the equipment, such as the boom, continues traveling or swing operation.
As described above, if the spool of the main control valve 6 is in a specific condition (e.g., if the stick of the spool or valve inferiority occurs), the equipment may malfunction regardless of the operator's intention, and this may cause another operator or building surrounding the equipment to be secondarily damaged.